Conventional optical fibers are made from glass materials and widely used as optical signal-transmitting mediums for instrumentation between instruments or in an instrument, for transmission of data, for medical use, for decoration, for transmission of image, etc. However, they are poor in flexibility when their diameters are not small enough. Further, they are relatively fragile and are apt to be broken by impact. Furthermore, they are heavy, because their specific gravity is comparatively large. In addition, the optical fibers themselves as well as their connectors are expensive. Due to these drawbacks, attempt has been made to replace glass materials with plastic materials.
The advantages with plastic materials are numerous. For instance, the resulting optical fibers are light, tough and flexible so that their diameters and numerical apertures can be made large. Further, for instance, their handling is easy and can be readily connected to light emitting and/or accepting elements. In general, a plastic optical fiber comprises a core made of a plastic material having a larger refractive index and a good optical transmission and a cladding made of a plastic material having a smaller refractive index and a high transparency. In this structure, light is transmitted by reflection at the interface between the core and the cladding. A larger difference in refractive index between the plastic materials of the core and of the cladding provides the optical fiber with better optical transmission. As the plastic materials having good optical transmission, amorphous plastics are preferred, examples of which are polymethyl methacrylate, polystyrene, etc. (cf. Japanese Patent Publication No. 8978/1968).
The plastic optical fiber, however, produces a huge decrease of optical transmission with an elevation of temperature and would accordingly, deteriorate the reliability of the fiber as a light signal-transmitting medium. In addition, it is insufficient in heat resistance so that its use in transporting vehicles such as automobiles, trains, vessels, aircrafts, robots, etc. is quire restricted. The maximum temperature at which plastic optical fibers made from polymethyl methacrylate and polystyrene can be used is about 80.degree. C. When used at a temperature higher than about 80.degree. C., these fibers become deformed and their microstructures become fluctuated; and the function thereof as a optical fiber is thus damaged. Once these fibers are used at a temperature higher than 80.degree. C., the attenuation of light transmission is great even after cooling to room temperature, and these fibers can be used only within a very restricted temperature range. Accordingly, a plastic optical fiber having good heat resistance is highly desired.
Polymethyl methacrylate has a larger water absorption capability than polyolefins and polystyrene. For example, polymethyl methacrylate absorbs more than 2% by weight of water at a temperature of 100.degree. C. and a relative humidity of 100% when measured according to ASTM D-570. When the material of the plastic optical fiber absorbs more than 1.8% by weight of water, it suffers dimensional change, warpage, or cracks which results from repeated cycles of wetting and drying. Thus, the use of a plastic optical fiber comprising polymethyl methacrylate is restricted in certain circumstances and/or application fields. Thus, improvement of the hygroscopic property of the plastic optical fiber is also desired.
Japanese Patent Kokai Publication (unexamined) No. 221808/1983 discloses a plastic optical fiber with good heat resistance and optical transmission comprising a core and a cladding, wherein the core is made of a polymer comprising units of a methacrylate ester the ester moiety of which has an alicyclic hydrocarbon of at least 8 carbon atoms, and the cladding is made of a transparent polymeric material having a refractive index of at least 3% smaller than that of the core. Some of the cladding materials used in the plastic optical fiber of the above publication is, however, desired to be improved in long-time heat resistance, humidity resistance and attenuation of light transmission although they have satisfactory short-time heat resistance.